In radiation therapy, ionizing radiation is used for medical purposes, in particular as part of cancer treatment to control or kill malignant cells. The most common type of radiation therapy is based on x-ray photons. MeV photons can be generated with moderate effort using for example a linear accelerator for accelerating an electron beam that is directed to a target to generate x-ray radiation as Bremsstrahlung. However, in x-ray radiation therapy, one always has to cope with the problem that the dose cannot be confined to the tumor area, but also affects healthy tissue. Part of this problem is due to the fact that the energy deposition of an x-ray beam in human tissue decreases nearly exponentially along the penetration depth and that the x-ray radiation will also affect healthy tissue in front of and behind the target area, since the x-rays are typically applied from outside.
In contrast to this, using charged particles, such as protons, boron ions, carbon ions or neon ions, the damage to healthy tissue can be significantly reduced. To appreciate this, reference is made to FIG. 1, where the energy deposition or dose of a 10 MeV photon as a function of penetration depth is compared with that of several proton beams of different energies. As is seen in FIG. 1, the so-called Bragg curve of the energy loss versus penetration depth shows a pronounced peak immediately before the protons come to rest. In the art these peaks are referred to as “Bragg peaks”. The penetration depth or location of the Bragg peak depends on the energy of the proton beam: The higher the energy, the larger the penetration depth. By combining a plurality of proton beams with different energies, a dose as shown by the envelope curve in FIG. 1 can be obtained, which has a maximum in a certain target region, which is below this maximum between the ion source and the target region and which drops sharply to zero behind the target region. Accordingly, using protons or other ions in radiation therapy, an adverse effect to healthy tissue in front of and behind the tumor region can be drastically reduced as compared to x-ray radiation.
While ion radiation therapy therefore has the capability of a more precise targeting of the tumor as compared to x-ray radiation therapy, the beneficial effect clearly depends on whether one is able to precisely deliver the dose to the target region as planned. This in fact remains one of the current challenges of ion radiation therapy. In particular, it is currently not possible to predict the precise location of the Bragg peak along the ion beam axis with a desired precision, which precision needs to be particularly high if organs at risk are located just behind the target region. Accordingly, there is a need to monitor the actual dose delivered during the therapeutic irradiation (“in vivo”).
Among the recent attempts to measure the dose in vivo are those related to secondary radiation employing, e.g. positron emission tomography (PET) and gamma imaging, see for example Ben Mijnheer et al., “In vivo dosimetry in external beam radiotherapy”, Med. Phys. 40 (7), July 2013 and Antje-Christin Knopf and Antony Lomax, “In vivo proton range verification: a review”, Phys. Med. Biol. 58 (2013) R131-R160.
However, these recent methods rely on complex instrumentation, and they do not allow for a straightforward correlation between the secondary radiation and the real dose distribution, so that a Bragg peak positioning accuracy better than a few millimeters cannot be expected in clinical situations.
In J Tada et al., “Time Resolved Properties of Acoustic Pulses Generated in Water and in Soft Tissue by a Pulsed Proton Beam Irradiation—A Possibility of Dose Distribution Monitoning in Proton Radiation Therapy”, Med. Phys. 18 (6), 1991, time-resolved acoustic pulses were generated in water and soft tissue by pulsed proton beam irradiation. The spatial resolution of depth dose distribution at the clinically applied beam intensity using time-of-flight measurement was estimated to be about 3 mm. In following experiments by the same research group, acoustic pulse signals have been observed even during therapy, see Y. Hayakawa et al., “Acoustic Pulse Generated in a Patient During Treatment by Pulsed Proton Radiation Beam, Radiation Oncology Investigations”, 3 (1995), 42-45.
However, while this method for monitoring dose distribution has been proposed more than 20 years ago, it has so far not led to any practical applications.